Shield volcanoes of Marie Byrd Land, West Antarctic rift: oceanic island similarities, continental signature, and tectonic controls

Abstract

The Marie Byrd Land volcanic province is largely defined by 18 large (up to ~1,800 km3) alkaline shield volcanoes, each surmounted by a summit section of varied felsic rocks dominated by trachytic flows. They are distributed over a 500 × 800-km block-faulted dome within the West Antarctic rift. The basement contact of volcanic sections is ~500 masl at one site and 3,000 mbsl at another, 70 km away, which illustrates the scale of block faulting but complicates an understanding of volcanic structure. Furthermore, the continental ice sheet buries 16 volcanoes to progressively greater heights inland. However, five are sufficiently exposed to allow meaningful comparisons with alkaline oceanic island volcanoes; these comparisons are used as a guide to estimate the structure of Marie Byrd Land volcanoes. The type example for this study is Mt. Murphy, the most completely exposed volcano. It consists of a 1,400-m section of alkaline basalt overlain by trachyte and benmoreite flows that make up ~7–13 % of the volcano volume. In gross structure and composition, Mt. Murphy is similar to Gran Canaria volcano, Canary Islands, but the percent of felsic rock may be three times that of Gran Canaria, if the estimate is approximately correct. Departures from the oceanic island example are believed to represent the imprint of the Marie Byrd Land lithosphere and tectonic environment on volcano evolution. These include a lack of order in the sequence of felsic rock types, lack of progression toward more silica undersaturated compositions with time, absence of a highly undersaturated mafic resurgent stage, and perhaps, a relatively large volume of felsic rock.